The interest in optical biological and chemical sensing technologies has never been higher than in recent years. The demand for increased sensitivity and parallelism has arisen not only from areas of pure research, such as the burgeoning field of proteomics, but also from the pharmaceutical industries due to its utilisation in drug discovery processes. A wide range of optical methods are exploited in bio-chemical sensors including interferometry (e.g. Cush R, Cronin J M, Stewart W J, Maule C H and Mollow J 1993 The resonant mirror: a novel optical biosensor for direct sensing of biomolecular interactions Part I: Principle of operation and associated instrumentation Biosens. Bioelec. 8 347-354), spectroscopy in optical waveguides (e.g. Heideman R G, Kooyman R P H and Greve J 1993 Performance of a highly sensitive optical waveguide Mach-Zehnder interferometer immunosensor Sens. Actuators B. 10 209-217), fluorescence spectroscopy (e.g. Rowe-Taitt C A, Hazzard J W, Hoffman K E, Cras J 5, Golden J P and Ligler F S 2000 Simultaneous detection of six biohazardous agents using planar waveguide array biosensor Biosens. Bioelec. 15 579-589) and surface plasmon resonance (SPR) (e.g. Homola J 2003 Present and future of surface plasmon resonance biosensors Anal. Bioanal. chem. 377 528-539 or Nylander C, Liedberg B and Lind T 1982 Gas detection by means of surface plasmon resonance Sens. Actuators. 3 79 (1982)). Fluorescence spectroscopy offers ultra-high sensitivity but requires the use of fluorescent labels, which is frequently undesirable. On the other hand interferometric, waveguiding and SPR techniques have the advantage of being label free. Additionally, they allow many reactions to be studied in real-time, allowing the reaction binding kinetics to be quantified in detail.
Surface plasmon polaritons (SPPs) (commonly called surface plasmons) are localized electromagnetic fields coupled to charge density oscillations at the interface of a metal and dielectric (see Raether H, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer, Berlin, 1988)). Surface plasmon resonance (SPR) sensors utilise the property that the surface plasmon polariton (SPP) is sensitive to changes in the local refractive index of the dielectric. The evanescent fields of the SPP decay into both the metal and dielectric media, with a decay length in the dielectric which is typically less than half the incident wavelength. For a planar interface the in-plane wavevector of the SPP, kspp, is given by
                              k          spp                =                              k            0                    ⁢                                                                      ɛ                  m                                ⁢                                  ɛ                  d                                                                              ɛ                  m                                +                                  ɛ                  d                                                                                        (        1        )            where ∈m and ∈d are the permittivities of the metal and dielectric media respectively and k0 is the wave vector in free space. Any change in the local refractive index and therefore the permittivity (∈d), either by way of a bulk index change, or, as for instance in the case of a biosensor, by the binding of an analyte to the SPP active interface thus changes the SPR excitation conditions. Various methods of probing these changes are utilised in SPR sensors such as angle (Matsubara K, Kawata S and Minami 1988 Optical chemical sensor based on surface plasmon measurement Appl. Opt. 27 1160-1163), wavelength (Zhang L M and Uttamchandani D 1988 Optical chemical sensing employing surface plasmon resonance Electron. Lett. 23 1469-1470) and phase (Nelson S G, Johnston K S and YEE S S1996 High sensitivity surface plasmon resonance sensor based on phase detection Sens. Actuators B. 35-36 187-191) intenogation, with varying degrees of sensitivity, and multiplexing capabilities, dependent upon the exact configuration, with the most sensitive methods allowing RI sensitivities of the order of 10−7 Refractive Index Units (RIU).
By controlling the surface chemistry at the SPP active interface a generic SPR sensing system can be tailored allowing a large range of different analytes to be monitored. Examples of detection studies found in the literature include: the monitoring of the pesticide atrazine in water, where real time analysis was undertaken by Minunni et al. and a detection limit of 0.05 ng mL−1 was determined (Mininni M and Mascini M 1993 Detection of pesticide in drinking water using real-time biospecific interaction analysis (BIA). Anal. Lett. 26 1441-1460), the detection of concentrations of morphine as low as 0.1 ng mL−1 obtained by Sakai et al. (Sakai G, Ogata K, Uda T, Miura N and Yamazoe 1998 N A surface plasmon resonance-based immunosensor for highly sensitive detection of morphine Sens. Act. B. 49 5-12), a concentration limit of 0.1 ng mL−1 of methamphetamine using a SPR based biosensor developed by Sakai et al. (Sakai G, Nakata S, Uda T, Miura N and Yamazoe N 1999 Highly selective and sensitive SPR immunosensor for detection of methamphetamine Electrochimica Acta. 44 3849-3854), a lowest detection limit of 6 μg mL−1 of E. coli by Spangler et al. (Spangler B D, Wilkinson E A, Murphy J T and Tyler B J 2001 Comparison of the spreeta surface plasmon resonance sensor and a quartz crystal microbalance for detection of escherichia coli heat-labile enterotoxin Anal. Chimi. Acta. 444 149-161) and Choi et al. used a commercial SPR sensor produced by Biacore (Biacore X) to detect botulinum toxin in concentrations as low as 2.5 μg mL−1 (Kibong C, Wonjun S, Seunghee C and Jungdo Choi 1998 Evaluation of two types of biosensors for immunoassay of botulinum toxin Biochem. Mol. Bio. 31 101-105).
It is clear from this small sample of the literature that SPR sensors are regularly used in a wide range of fields including environmental analysis, medical diagnostics, food safety etc, as well as the previously mentioned drug discovery. However, their use is not limited to detection studies; they are also regularly used in research studies for subjects such as proteomics and surface chemistry. Examples include: Liu et al. used a SPP based sensor to measure the length of DNA with sub nanometre axial resolution (Gang L et al. A Nanoplasmonic Molecular ruler for measuring nuclease activity and DNA footprinting Nat. nanotech. 1 47-52 (2006)), Campagnolo et al. used protein marker detection of tumour-antigen and serum-antibody interactions, monitored in real time using a SPR sensor (Campagnolo C, Meyers K J, Ryan T, Stkindon R C, Chen Y T, Scanlan M L, Ritter G, Old L J and Batt C A 2004 Real-time, label-free monitoring of tumor antigen and serum antibody interactions J. Biochem. Biophys. Methods. 16 283-298), and Chou et al. developed a ferritin (a non-specific tumour marker) immunosensor using SPR sensing analysis (Chou S F, Hsu W L, Hwang J and Chen C Y 2004 Development of an immunosensor for human ferritin, a non-specific tumor marker, based on surface plasmon resonance Biosens. Bioelectron. 19 999-1005).
Polarisation of the incident light is discussed in I. R. Hooper, J. R. Sambles, “Sensing using differential surface plasmon ellipsometry”, Journal of Applied Physics, Volume 96, Number 5 (September 2004), pp. 3004-3011; and in I. R. Hoooper, J. R. Sambles, “Differential ellipsometric surface plasmon resonance sensors with liquid crystal polarization modulators”, Applied Physics Letters, Volume 85, Number 15 (October 2004), pp. 3017-3019.
SPR sensing techniques are constantly being developed and refined in order to meet the increasing performance demands required. As set out in Homola J 2003 Present and future of surface plasmon resonance biosensors Anal. Bioanal. chem. 377 528-539, there are 3 main avenues of research being focused upon, 1) increasing the sensitivity, 2) miniaturisation, so that SPR sensors can be utilised in the field and 3) increasing the number of simultaneous sensing channels.
Further background reading includes WO2008/007115, US2007/0216901, US2007/0159629, WO2007/061981, EP0341927 and Nylander et al “Gas Detection by means of surface plasmon resonance” T Sens Actuators, 3, 79-88 (1982).